Efficient transformation and expression of Vitreoscilla haemoglobin in the biological control bacterium Collimonas pratensis ZL261

Collimonas species are soil bacteria characterised by their ability to attach to and utilise fungi as a food source (mycophagy), as well as their chitin-degrading capacity (via chitinase production). These attributes, alongside volatile compounds, are thought to contribute to their function as fungal antagonists, including economically important plant pathogens. Despite this, studies have found no relationship between antifungal activity and chitinase production, or volatile compounds in Collimonas pratensis isolate ZL261, and there have been no studies on genetic control and regulatory biosynthesis of antifungal substances in Collimonas species. In this study, we showed that low concentrations of dissolved oxygen were unfavourable for growth and antifungal activity. We successfully introduced the gene vgb encoding Vitreoscilla haemoglobin (VHb) into isolate ZL261. The heterologous expression of VHb not only enhanced cell growth, but also improved antifungal activity against the brown rot fungus Monilinia fructicola under oxygen-restricted conditions; 18.6% of untreated peach fruits were infected (average lesion diameter: 9.2 mm), while only 10.8% of fruit treated with the transformed isolate, ZV261, were infected (average lesion diameter: 5.4 mm). These results suggest that the antagonism have been due to the secreted secondary metabolites, which are sensitive to the oxygen-restricted conditions.